The most commonly used material for making stopper-type bottle closures is natural cork. There are, however, significant disadvantages to using cork in the manufacture and marketing of bottle closures. Cork has variable properties with respect to color, drying, shrinkage or expansion, crumbling, sticking to containers and seal formation. These features are generally unsatisfactory in terms of production and consumer costs as well as product performance. In the case of wine closures, cork may also impart an odor to the product, causing it to be rejected by consumers. In addition, nearly 10% of bottled wine is discarded because of unpredictable contamination by mold from natural cork. Further, the use of cork for producing bottle closures is becoming increasingly expensive as the supply of trees from which cork is obtained rapidly diminishes.
Numerous attempts have been made to develop alternatives to natural cork bottle stoppers. Among these, screw top closures for wine containers have been found largely unsuitable because they do not provide the appearance, ceremony or romance that surrounds traditional cork wine closures. A number of synthetic cork closures have also been developed. In particular, recent efforts to develop closures from injection molded foam thermoplastics have encountered numerous pitfalls, particularly in terms of production costs, product performance, and consumer acceptance. Some of these closures have exhibited a tendency to noticeably taint the product and/or offer low resistance to oxygen permeation into the container. In addition, synthetic closures from foam thermoplastics have generally exhibited poor uniformity in terms of size, shape, weight, and other features important to production, marketing, and performance.
In a particular case of injection molding of foam thermoplastic closures for liquid containers, thermoplastic compositions are injected into relatively cool molds, leading to the formation of a dense outer "skin" at the surface of the closure and a porous, foam-like interior. The composition of the closure and the structural relationship between the outer skin and porous core of the closure may be critical to the sealing capabilities of the finished closure. Other foam thermoplastic closures have suffered problems due to wrinkling of the outer skin layer, which can produce leakage fissures between the closure and container. Other prior art closures have different sealing problems, for example, failing to quickly return to normal size after compression, such that reliable seal formation requires containers to be kept in an upright position for an extended period after insertion of the closure. Further, prior art closures are unable to significantly retain printed matter for a significant period of time.
U.S. Pat. No. 4,363,849 discloses the production of thermoplastic closures having a natural cork-like appearance. Special molding apparatus are used, however, and it is necessary to gradually release the air in the cold molding cavity that is displaced by the injected thermoplastic resinous material by means of controlled minimum venting or other means in order to maintain a heightened pressure, for example about 16,000 psi, within the mold. U.S. Pat. No. 4,188,457 discloses a thermoplastic composition that is also used to form closures for wine bottles. The thermoplastic composition includes sulfur dioxide and water, and the sulfur dioxide is said to act as an oxygen scavenger. This method is undesirable, however, because the sulfur dioxide or the metabisulphite from which it is produced may taint the liquid in the closure.